The completion of the Arabidopsis thaliana genome sequence allows a comparative analysis of transcriptional regulators across the three eukaryotic kingdoms. Arabidopsis dedicates over 5% of its genome to code for more than 1500 transcription factors, about 45% of which are from families specific to plants. Arabidopsis transcription factors that belong to families common to all eukaryotes do not share significant similarity with those of the other kingdoms beyond the conserved DNA binding domains, many of which have been arranged in combinations specific to each lineage. The genome-wide comparison reveals the evolutionary generation of diversity in the regulation of transcription.

The chloroplast signal recognition particle (cpSRP) is a protein complex consisting of 54-and 43-kD subunits encoded by the fifty-four chloroplast, which encodes cpSRP54 (ffc), and chaos (cao) loci, respectively. Two new null alleles in the ffc locus have been identified. ffc1-1 is caused by a stop codon in exon 10, while ffc1-2 has a large DNA insertion in intron 8. ffc mutants have yellow first true leaves that subsequently become green. The reaction center proteins D1, D2, and psaA/B, as well as seven different lightharvesting chlorophyll proteins (LHCPs), were found at reduced levels in the young ffc leaves but at wild-type levels in the older leaves. The abundance of the two types of LHCP was unaffected by the mutation, while two others were increased in the absence of cpSRP54. Null mutants in the cao locus contain reduced levels of the same subset of LHCP proteins as ffc mutants, but are distinguishable in four ways: young leaves are greener, the chlorophyll a/b ratio is elevated, levels of reaction center proteins are normal, and there is no recovery in the level of LHCPs in the adult plant. The data suggest that cpSRP54 and cpSRP43 have some nonoverlapping roles and that alternative transport pathways can compensate for the absence of a functional cpSRP.

Syntaxins are a large group of proteins found in all eukaryotes involved in the fusion of transport vesicles to target membranes. Twenty-four syntaxins grouped into 10 gene families are found in the model plant Arabidopsis thaliana , each group containing one to five paralogous members. The Arabidopsis SYP2 and SYP4 gene families contain three members each that share 60 to 80% protein sequence identity. Gene disruptions of the yeast ( Saccharomyces cerevisiae ) orthologs of the SYP2 and SYP4 gene families (Pep12p and Tlg2p, respectively) indicate that these syntaxins are not essential for growth in yeast. However, we have isolated and characterized gene disruptions in two genes from each family, finding that disruption of individual syntaxins from these families is lethal in the male gametophyte of Arabidopsis. Complementation of the syp21-1 gene disruption with its cognate transgene indicated that the lethality is linked to the loss of the single syntaxin gene. Thus, it is clear that each syntaxin in the SYP2 and SYP4 families serves an essential nonredundant function. INTRODUCTIONThe syntaxins are a large evolutionarily conserved family of proteins required for fusion of transport vesicles in eukaryotic cells. Individual syntaxins reside on the organelles of the endomembrane system in which it is believed that they assemble with other proteins of the SNARE (for soluble N -ethylmaleimide sensitive factor attachment protein receptor) family to form t-SNARE complexes. The t-SNARE complex serves as a binding site for v-SNAREs on the transport vesicle, and association of the v-SNARE with the t-SNARE complex is thought to drive membrane fusion (McNew et al., 2000).The Saccharomyces cerevisiae genome encodes eight syntaxins (Pelham, 1999), two of which, Sso1p and Sso2p, are functionally redundant (Aalto et al., 1993). Gene disruptions of all yeast syntaxins have been characterized, with only the ER syntaxin Ufe1p, the Golgi syntaxin Sed5p, and one of either Sso1p or Sso2p (plasma membrane syntaxins) being essential for viability, although mutations of the other syntaxins result in large defects in vesicle trafficking (reviewed in Pelham, 1999). The effect of the loss of a syntaxin gene in a multicellular eukaryote has been examined in only a few cases. The Caenorhabditis elegans unc-64 mutant, which corresponds to a plasma membrane syntaxin, has pleiotrophic defects in coordination and neurotransmitter release (Ogawa et al., 1998). Mutants in Drosophila melanogaster syntaxin 1 can be examined in flies that are mosaics of mutant and wild-type cells; these flies show multiple defects in many organs, including a failure to complete cellularization in the eye (Schulze and Bellen, 1996;Burgess et al., 1997). In the plant Arabidopsis thaliana , mutations in the KNOLLE gene, which encodes a syntaxin (now called SYP111; see Sanderfoot et al., 2000) involved in cytokinesis, lead to defects in the embryo, eventually resulting in seedling lethality (Lukowitz et al., 1996).The genome of Arabidopsis encodes 24 syntaxins that we h...